1. Field of the Invention
The present invention relates to a mold for shaping a golf ball with excellent symmetry, and a ball shaped by the mold.
2. Description of the Related Art
As golf balls, a rubber thread type produced by covering a core formed by spherically winding a rubber thread with a balata cover, and a solid type produced by covering a rubber spherical core with a resin cover are commonly known. Among them, the solid type is prevailing among golfers.
How a solid golf ball is shaped will be described with reference to a cross-sectional view of a mold shown in FIG. 4.
In FIG. 4, a mold 1 comprises an upper mold 2 and a lower mold 3 to be parted at a parting surface P, with a spherical cavity 4 formed therein. The cavity 4 has a large number of projections (not illustrated) with a circular shape when viewed two-dimensionally on an inner wall surface 5 for forming dimples. The parting surface P is provided at a position corresponding to an equator of the spherical cavity 4. In the figure, the cavity 4 accommodates an elastic core 12 in the center thereof, and the elastic core 12 is supported by a plurality of support pins 6 from above and below. A projection for forming a dimple which is the same or similar to at least one dimple one provided on the cavity wall surface 5 is provided on the tip portion of each support pin.
Reference numeral 7 denotes a runner provided along the circumference of the cavity 4. A thermoplastic resin for the covering material is pressed into the space between the core 12 and the inner wall surface 5 through a plurality of (4 to 12) gates elongating from the runner. Immediately before the covering material is cured, the support pins 6 are withdrawn to the level of the cavity inner wall surface 5 so that the covering material is shaped thereafter by cooling. At that time, the dimples at the positions of the support pins 6 are formed by the tip portions thereof. Reference numeral 9 denotes a degassing means for discharging a gas existing or generated in the cavity at the time of shaping.
Incidentally, a plurality (in general three to four pieces) of support pins are perpendicular to the parting surface P of the mold and provided on the circumference with a 7 to 9 mm radius, centering on a line j linking both poles of the cavity. With the support pins disposed accordingly with respect to the mold, the cross-section of the support pins 6 in the direction perpendicular to the axial direction thereof is a round shape with a diameter x in the direction of the central axis j linking both poles of the cavity and a diameter y in the direction perpendicular thereto equally provided as shown in FIG. 5. However, the cross-section in the direction of the normal of the wall surface in the cavity is an elliptic shape with the diameters xxe2x80x2 and yxe2x80x2 corresponding to the diameters x and y mentioned above provided so that xxe2x80x2 greater than yxe2x80x2 as shown in FIG. 6.
Apparently, the dimples shaped by the elliptic projections at the support pin tip portions are elliptic dimples different from the round dimples formed in the other area.
A golf ball having both round and elliptic dimples is poor in terms of not only the external appearance but also the flying symmetry according to the rotational direction of the hit ball.
In order to solve the above-mentioned problem, an object of the invention is to provide a golf ball mold capable of shaping the same dimples at the positions of support pins as in the other portions, and a golf ball shaped by the mold.
In order to achieve the above object, the invention provides a golf ball mold comprising: an upper mold and a lower mold for forming a spherical cavity having a large number of projections for shaping dimples on an inner wall surface thereof with a parting surface provided at a position substantially corresponding to an equator of the cavity; and a plurality of core support pins elongating reciprocally movably in a direction perpendicular to the parting surface on a circle centering on each pole of the cavity of the upper mold and the lower mold with a predetermined radius with a distance with each other in the cavity, wherein the support pins have a dimple shaping projection at a tip portion thereof having a round shape on a cavity wall surface level, protruding from the position to a cavity center direction when the support pins are withdrawn to a cavity wall surface position.
Further, the invention provides a golf ball shaped by the above-mentioned golf ball mold.
It is preferable that the support pins of the mold have an elliptic cross-sectional shape in the direction perpendicular to the central axis thereof with the minor axis disposed in the direction of the axial line linking both poles of the cavity.
Moreover, it is preferable that the support pins centering each pole of the cavity are disposed at 3 to 5 positions on a circle with a 6 to 10 mm radius centering on the axis linking both poles of the cavity with an equal interval.
Furthermore, it is preferable that the tip portion projections of the support pins to be disposed on the circle have the same size. However, a plurality of pairs of projections with different projection sizes can be used with the ones with the same size disposed at positions with a 180xc2x0 angle with each other. Moreover, ones with different projection sizes can be used in a combination as long as the object of the invention is achieved.